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How do you determine how much "life" an aircraft has left?
I am looking to purchase an aircraft, and I've found one described as follows

Total time is 6300 hours
Engine time since major overhaul 2100 hours
  Engine runs strong and compression is all in the 70s
Has VAL Avionics all-in-one VOR/LOC/GS unit

The owner also states that the engine was rebuilt 556 hours ago, but this was never entered in the log books, so we're working based on 2100 hours since overhaul.
We have done a pre-buy inspection through A&P mechanic. On the report was mentioned,

TACH TIME ;8131.18
  HOBBS ;  7141.9

Can you explain for me what these time figures mean and how I can judge the remaining life left for this engine and airframe?

Why wouldn't you just ask the mechanic for his opinion on the engine life?  Having been the only one to actually inspect it, he's the only one qualified to answer.

The thing is I dont have direct contact with mechanic.I got only the inspection report.I am a biginer for the aircraft trade.hence I realy need to know the sellers mention of the aircraft detal which I had forwerded to U.thanks,

@Fernado Are you sure you want to spend that much money on something you don't seem to know much about? Holding back and doing some research sounds like a good idea.

doesn't the inspection report contain the details of the executing mechanic?

TT vs tach vs Hobbs are all about a 1000 hrs different. Engine was rebuilt but wasn't entered in the maintenance logs?  What else has not been entered?  Don't go near it.

Tach and Hobbs can often be 10-20% off, especially if Hobbs runs off the master

Not necessarily recommended, but I know Lycoming O-320 engines that have been run to 4-8k hours. Time isn't everything.

I'm going out a bit on a limb here, and it isn't specifically an answer to exactly what you are asking (which is what Stack Exchange tends to strive for), but I think this is the answer you need, so at the risk of receiving a couple of downvotes, here goes.

engine was rebuilt 556 hours ago but was a problem because was never entered in the log books

I think that boldfaced text makes your entire question moot. In other words, the answer to your specific question doesn't really make any difference; the above tells you all you really need to know. Which is:
As Simon said in a comment, if the seller readily admits to not entering an engine rebuild into the log, then what else has not been entered into the log? Especially if you are a beginner, that's the sort of thing I would expect will come back to bite you hard.
Total time is stated as 6300, Hobbs time is stated as 7142, tach time is stated as 8131. And that's by the seller (including the mechanic's report provided by the seller). Does the difference between these figures inspire confidence in the airplane's state, the seller's honesty, and your ability to get an accurate picture of what you are buying?
As has been stated, the only person qualified to answer your question about the state of the engine in a reasonably objective manner is the mechanic who inspected the engine. The odds that this particular mechanic will stumble across this question is already quite low. What are the odds that the mechanic will not only stumble across the question, but also recognize the particular aircraft you are asking about and decide to answer the question with anything that is not already in the inspection report (which you already have received from the seller)?
More generally, with well-serviced aircraft, it isn't as easy as just saying "this aircraft has X number of hours of service left" even based on the current state of a single piece such as the (an) engine, because with good servicing it's perfectly possible to extend the service life of an aircraft for a very long time. A well-treated aircraft is a very different beast compared to perhaps a car (which might be lucky to get a checkup once a year and an oil change every few thousand miles), of course in part because the failure mode is much more serious. (If your car fails on the highway you pull over and call a tow truck; if an airplane fails in mid-flight then what do you do?)
Personally and for the specific situation you describe, I wouldn't go near that particular aircraft if I can avoid it, let alone buy it. At the very least buy something where the trivially obtainable numbers match.

A difference between tach, hobbs, airframe, and engine time is not at all unusual in older aircraft where components have been replaced. Case in point, mine has 4000 airframe hours, and about 425 since the last engine overhaul. The tach reads around 2000 hours (because it was replaced and they didn't run the new tach up to match the airframe time), and if I installed a Hobbs meter today it would start from 0 hours.

@voretaq7 I can certainly see the point of your comment, but there is also the "little detail" that the engine rebuild (whatever exactly that means) was never entered into the log. That would be the major killer in my book.

Agreed - an engine rebuild is something that would have had to be done by the factory and would certainly be in the logs (the engine log would have been reset), so the fact that it's not "in the logs" is a HUGE red flag.

@voretaq7 There is also the (perhaps likely given the display name) possibility that English is not the OP's native language, and it really was "just" an engine overhaul done by a local mechanic. Even so, that too should be in the log, so the question posed in the 4th paragraph is valid either way. Hence, even in that case, I still stand by this answer.

I'll echo many of the sentiments that have already been expressed: There's a discrepancy between what the ad describes and what the mechanic's prebuy report describes, which indicates that either the ad is "hastily put together" or intentionally misleading (neither is good). Also the idea that an engine was "rebuilt" but never logged is a huge red flag.
Were I in your situation (and I was not too long ago) I would only consider purchasing this aircraft if you could talk to the inspecting mechanic in person (preferably while seeing the aircraft), and I would ask their opinion on the condition of the airframe, engine, accessories, instruments, and avionics.
In fact that is very good advice no matter how experienced you are at purchasing aircraft: Nothing beats an expert evaluation.

To answer your question about the remaining life on an airframe and engine requires a deeper understanding of the plane you're looking to buy (again, talk to your mechanic about this - I really can't emphasize that enough).
You'd be best served by first understanding what Tach and Hobbs hours mean, and how engine time is described. Armed with that knowledge, let's forge on and figure out what times are important for this particular plane.
First you'll notice that the Tach and Hobbs times in the mechanic's report are different -- that's OK, they're probably never going to be exactly the same. You'll also notice that none of these numbers match the "Time Since Major Overhaul" for the engine -- that's also normal (TSMOH is a derived number - the current time minus the time when the last overhaul was done - so there's almost never a direct representation of it in the cockpit).
You'll also notice the Tach and Hobbs, and Total Time (airframe time) don't match up with the total time for the airframe. That's a bit of a red flag: Normally one of these is what's being recorded as "Total Time - Airframe" (TTAF), and if none of them match either the tach / hobbs meter was replaced at some point and not "run up" to match the airframe time, or there's a error in the ad as I mentioned earlier.
Your mechanic should be able to tell you which situation you're in by a careful examination of the logs, and that should also reveal how many hours are on the engine and airframe.
For the sake of argument I'm going to assume the Tach time above is the Total Time - Airframe (about 8100 hours), and we'll go with 2100 hours since overhaul on the engine because that's what the logs apparently say.

So we've established the Total Time for the airframe (about 8100 hours), and the time since the engine was last overhauled (2100 hours) -- what does that mean? How long will this plane last?
The short answer is there's no way to know just based on hours -- this is why we pay mechanics to do a thorough prebuy inspection. A 20,000 hour airframe that has had excellent maintenance could easily go another 20,000 hours, but a 2,000 hour airframe that has been abused and neglected may be ready to be scrapped.
For the engine, most GA Piston engines have a 2000 hour Time Between Overhaul (TBO) recommended by the manufacturer. Overhauling at this point isn't mandatory for most part 91 operations and well-cared-for engines can continue to operate for hundreds of hours beyond the recommended overhaul point with no problems, but generally if you're looking to buy an airplane an engine that's got 2100 hours on it will be considered "run out". Consider your mechanic's opinion of this engine carefully, and have a plan in your mind as to what you're going to do when the engine needs to be torn down.
For the airframe 10,000 hours is usually considered to be "high time" (often these have been "working airplanes" - flight school trainers, etc.) - This doesn't mean the plane is worn out, but it bears special consideration in the inspection.
Remember that nothing can ever "reset" airframe time the way overhauls or engine replacement can for an engine: The parts on that airframe have their accumulated time (and accumulated fatigue cycles) forever.
A 10,000 hour airframe can continue to give years of excellent service, with one major caveat: Life-Limited Parts.  Some aircraft have components which have a maximum service life limit (for example, the wings on a Grumman AA-5 have a 12,000 hour life limit. Once reached the wings are no longer airworthy and must be replaced, and as Grumman is no longer making those planes you'd be hard pressed to find a replacement wing).
In consultation with your mechanic you should consider the time remaining on any life-limited parts as well as the overall time on the airframe and its condition in order to ascertain how many more hours the plane can realistically fly for.
Sometimes the numbers can be mind-boggling: When I was plane shopping I looked at a Cherokee 140 with 10,000 hours on the airframe. It was a flight school trainer, but somehow managed to get through its life with no major damage and had been meticulously maintained by the schools which owned it over the years. My mechanic's opinion was that it could easily fly another 20,000 hours or more if it continued to get the same kind of lavish care, which is more hours than many GA pilots will ever have in their logbooks. (Piper says the spar is good for 30,000 hours per Service Bulletin 886, after which they want it inspected).
Other times the numbers can be soberingly obvious (like Grumman AA-5 with 11,500 hours - obviously a plane in the twilight years of its service life).

So what's the bottom line?
The aircraft you're describing has mid-to-high time airframe by most standards.
The engine is "run out" based on the time since it was last overhauled.
Both could continue to give years of excellent service, or they could fall apart tomorrow: The only way to have confidence either way is to find a mechanic who knows that particular type of aircraft, hire them, and have them go over the plane and logs. Then sit down with that mechanic and go over their findings.
Your mechanic will tell you if the plane is a gem or a lemon, and if they know the aircraft type they've got a better idea what to look for than you, I, or most folks kicking around on the internet would.

TL;DR: sit down with the mechanic who inspected the plane and go over their results with them. You'll be glad you did!


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Is it possible to get permission for a GA flight to land at Reagan National Airport?
If it is possible for a General Aviation flight to land at Reagan National Airport, what would you have to do?
If it's not, what kind of security clearance do commercial carriers and their pilots have to go though in order to land there?

Special training is needed for the DC area in general.

You might want to take a look at this question: What requirements are there to fly into KDCA?

The TSA operates the DCA Access Standard Security Program (DASSP), which allows general aviation flights originating at designated gateway airports to fly into DCA, however there are many restrictions in place, some of the most significant being:

You will need an FAA "reservation slot" to fly in to DCA
(This is required for all unscheduled flights into DCA)
You will need to submit a flight manifest & undergo TSA passenger/baggage screening
The flight crew will need to be cleared by the TSA (fingerprint/background check)
You will need to carry an Armed Security Officer onboard

So the answer is "Yes, you can do it, but you probably don't want to."
It also makes no sense to do it in something like a Cessna 172 as you're giving up at least one seat for the security officer. The program is really geared toward larger aircraft (business jets, etc.) transporting executives with a business reason to be in DC, like testifying before Congress.

If the pilot is themselves an Armed Security Officer, do they need to carry a second one on board?

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Why are US Regional Airline starting wages so low?
With 2,000hrs of flight time, a commercial ticket for ASEL and AMEL, CFI, CFII, MEI, and a Master's in Aeronautical Science I could easily qualify to fly right seat for a regional airline, but I just can't compete with pilots willing to work for \$17k/yr. For comparison, federal minimum wage is currently just over $15K/yr.
Why are the starting salaries at US Regional Airlines so low?
It seems odd that pilots go through extensive training which takes a great deal of time and expense, but they have to be prepared to work for what is a very low salary compared to other jobs that require a high level of skill and education.
Supply and demand is one explanation, but I really don't understand how there is so much competition for flying jobs but at the same time there is a supposed "pilot shortage".
Are there really so many pilots out there that the competition for jobs drives down starting salaries so low?
I am hoping to understand why it is that well-qualified pilots appear to have so little earning power in the marketplace.

It's worth noting that meaningful cost-of-living increases seem to have gone by the wayside sometime in the last 20 or 30 years. It's likely that the unions' concessions following deregulation in the mid 1980s (and their relative weakness since) were a factor.

There are some other ways some airlines choose to save money on new pilots. For example, do they pick up the cost of lodging during training, and do they pay during training. Of the four airlines I worked for, none of them paid during initial training, and only one paid for lodging during initial training. Also, none of them paid for travel expenses to get to the initial ground school.

This seems to be a simple problem of supply and demand.  Q: Wouldn't everyone love to get paid to fly around in planes, instead of doing some crappy desk job?  A: Yeah. lots of pilots + limited positions = low wages

So then why do they keep on advertising this "pilot shortage" that never comes? Or is that just an AOPA recruiting engine?

You have a TON of Baby Boomer pilots beginning to retire over the last few years. And there aren't any younger guys in line behind them to fill the ranks. Add to that the fact that the regs have recently changed to add significant time to be a 121 First Officer, and there's absolutely about to be a very visible shortage of qualified pilots. The pilot shortage has been here for a long time. It's just been camouflaged. If you have enough pilots to go around at all levels, then you never have to worry about "pilot fatigue" being an issue. Ask any regional pilot if that's the case.

Yet nobody sees the need to raise the pay..

@p1l0t There are more than a few factors involved there, and plenty of fault on all sides. Passengers, airline management and pilots are all culpable. But I do imagine that will begin changing a little bit here soon. It's more expensive for a pilot to be an airline pilot, and hopefully they'll be less willing to take the crappy pay and conditions when they realize they don't have to. Airlines will have to pay more to entice people to be pilots, especially in the US. And passengers will have to pay more to get from A to B safely. The whole industry is about 30 years behind schedule.

And I would still recommend against becoming an airline pilot. It would have to be an exceptionally more attractive career to entice me back towards it. Plus, there aren't very many careers that you can spend 10+ years in and then have to start over from scratch simply because you went to the same job at a different company. It would take some serious soul-searching to decide if it was worth it.

Oh yeah I love to fly and a big shiny jet sounds great but I need to earn a living wage. No way I'm moving halfway across the country for a 60+% pay cut. I would have to sell my house and I wouldn't even get another mortgage on that salary. Plus the pie in the sky used to be like $230k after 10yrs. Now it's barely 6 figures and you may never get there with everyone moving backwards. I guess it's like anything else though everyone is competing so hard with everyone else that they will all wait to move until they are forced even if it means that when they are the consequences will be dire.

Because the USA doesn't have a strong union movement. For example, Australian regionals start at about $55k, minimum wage is closer to $35k. The flip side to this though is that the airlines struggle financially, partly because of these wages.

Guys, does this mean USD$17,000 a month?

@Ben US airline unions are actually extremely powerful and have literally caused airlines to go bankrupt by launching strikes against companies that were financially incapable of meeting demands for pay rises of 30-50%.

@jwenting is that so? 30-50% pay rise?? That caused the airline to go bankrupt? Do you have an article or some other source for me to better understand the background of those demands and why the airline went bankrupt as a result?

You need to look at turn-of-the-century early industrial age history for your answer.  Factories were filled with skilled workers who could weave or smith or do some other specialty with great speed.  These were not unskilled workers by modern standards, but their skills were common in those days.  The efficient factories lowered prices for those products, making those skills less valuable.  The workers were dependent on that factory for their livelihood, they could not easily find work outside the factory.  If they grew up in that factory town, they likely had no other skills.  There were few factories of their kind they could work in, and no unions or labor laws.
Airline pilots are in a similar situation.  While airline mechanics can quit and maintain power plants or trucks with a little training, pilots are stuck with now three major airlines and a handful of regional airlines.  They have no skills that can cross over to another industry.  Their unions cannot strike or even hint at a job action expressing their discontent.  But the airlines can outsource their jobs to cheaper pilots, and have done so with over half their pilot jobs over the past decade.
Those cheaper pilots are disappearing, as the major airlines start hiring, and young people realize what the career of an airline pilot has become.  This has been leading up to a pilot shortage for many years, and the efforts to stave off that shortage have resulted in fewer pilots getting trained every year.  The result will certainly be a wave of much higher pay, but it won't change the underlying cause.  The wave of young people who rush into the career when the pay skyrockets will certainly see their pay drop again, along with layoffs and working the bottom jobs, when the airlines have enough pilots and hit another down decade.
Oh, and $17k is a little more than Skywest, Republic, American Eagle, Great Lakes, and other entry-level carriers pay their new-hires for the first several years.  Sorry.

Skywest is around $20k for the first year and increases thereafter, according to http://airlinepilotcentral.com

You entire economic history is backwards. 1) Industrialization increased the percentage of skilled technicians in society. Blacksmiths became mechanics. loomers because loom technicians etc. The unskilled labor came in off the farms and their kids became technicians and information workers. 2) Before union and outside of unions, industrialization raised effective salaries, especially when measured against purchasing power for essentials like housing and food. Your unknowingly repeating Marx's long discredited concept of ever downward spiraling wages. It never happened.

@egid and they probably charge you a good chunk of this in all kind of "fees" and "payback schemes" for training, lodging, supplies, etc. that in most any other industry the company would not charge to the employee.

Well, the short version is: The airlines pay what they have to pay in a free market.  They can get away with it because pilots are willing to accept it.  Like any other business they are in it to make money and keeping costs down is just as good as making extra money, so they aren't going to volunteer anything extra.
Now, why do pilots do it?  Because of the promise of better things down the road.  It isn't as good as it used to be, but after being at the airlines for a few years, you can make a livable wage and as you get further into your career at a major airline, you still get paid pretty well for a "part time" job.  As I've heard other pilots say before:  It's better than a "real" job!
Other pilots do it just because they love it and can't imagine doing anything else!

I'm not sure that the airlines pay what they have to pay - I think they probably do, however, pay what they want to pay.

@egid They have to pay it because of union negotiations and the law of supply and demand.

Ah, I see - your position is that, market permitting, they'd pay even less. Yep, can't disagree with that.

@egid Absolutely...  sigh  So would most of the fast food places too though, lol.

It's too bad down the road is so far away (especially with furloughs moving people backwards) that it still makes it a terrible 'investment.' I couldn't imagine moving halfway across the country for a 60+% paycut. I would have to sell my house and I wouldn't even be able to get a new mortgage with that salary. Maybe I should be thankful I can't compete with this.

@p1l0t: I agree, although hiring actually is turning around with so many people retiring...  I still think it's a dumb way to handle things....

I left the airline industry largely because of the bad pay and bad working conditions required to even get your foot in the door. Even today, I don't know that I could recommend an airline career to anyone. Things haven't changed a lot in the last decade. Other than the old guys kept getting older. Do I love flying? Absolutely. Is a wide-body Captain job at FedEx a good job? Absolutely there too. Is it worth it to spend over a decade working a high-skill, very-low-paying job in an industry that is incredibly cutthroat and willing to lay you off or replace you in a heartbeat? It wasn't for me.

There has been some hiring but who cares at what they are paying? When I was 22 I probably would have been happy to live in a crash pad in Detroit making $25k/yr just to say I have flown a jet. Now that I'm 31 and it's like $17k.. it's unbelievable. I had it easy, (almost no debt) but some of these kids have nearly $100k in loans between the ratings and the 4yr degree how do they even make the payments and still eat?

It still just comes down to supply and demand.  They pay what they pay because they keep finding pilots willing to do the job at that rate.  If/when the pilot shortage actually hits, then something is going to have to change and hopefully it will be a wage improvement for everyone.

@Lnafziger Hopefully the airlines figure it out before a serious pilot shortage actually hits. Good pilots aren't exactly something you can drum up in a few weeks.

There are probably several major effects.
1) Piloting is a "bikini inspector" job, one that people will do because they enjoy it and the social status it may convey. If you have a passion for a profession, you will work in it for less.
After all, people pay to pilot planes as a sport, so some percentage of pilots will work for relatively little pay just so they can fly. Those people making sub-optimum strictly economic decisions, pull down salaries in the field. You see that in other fields as well, such as Chefs.
2) Deregulation and globalization: From the early 1950s through the mid-1980s, airlines were run as government monopolies with government price fixing and protection from competition. Flying was a relatively high-margin business that catered to those flying on corporate money or who were individually wealthy ("the jet set.") People who chauffeur the rich tend to be better compensated.
The deregulation and opening of air travel to international markets drove down all costa as customers sought the best value. It has happened in every deregulated industry. Back when AT&T was a national government corporate monopoly, everyone, even down to the janitors, got above-market wages, unionized or not. After the monopoly broke up, wages went down.
In the past, airlines were semi-militarized and I know for some period back in the 50s and 60s, you had to be a US citizen to pilot commercial aircraft in the US.
3) As a field matures, its wages drop. Most technologies have a sideways-s growth curb where they start slow, have two to three decades of explosive growth and then plateau off. In the growth era, wages are high and then they regress relative to the general wage. In cars, it was 1920s-1960s; in computers, it was 1980s-now; in aviation, the explosive growth was 1930s-1970s.  (WWII threw things off some.)
4) People who can be pilots are not as relatively rare as they used to be. My grandfather was a mail pilot in the 1930s and WWII. Pilots back then had to be physically strong, immune to fatigue and able to do a lot of math on the fly (so to speak.) They were very extraordinary people and were often paid accordingly (e.g. clipper plane pilots made ridiculous amounts of money in the 1930s).
As planes gained larger crews and have become progressively automated, the potential talent pool in the general population has expanded. It's not just that more people can become pilots, it's that existing pilots can fly longer. In the 1950s, you didn't see 60 year old pilots. Now you do.
It's easier to become a pilot these days. When I was in my teens circa 1980, I wanted to get a civil aviation license, but couldn't because I was blind in one eye. Today, I can. An expanded talent pool decreases prices.
5) Pilots and aircrew in general are not as key as they once were. Now they are part of a larger team, many of whom stay on the ground.
I would also speculate that pilots just aren't as skilled, relative to the rest of the population, as they used to be. It's not just that the job is more automated and team driven, but that other jobs have reason to and have surpassed the technical complexity of piloting. Compared to other professions out there now, pilots are not the elites they once were. Their pay has regressed accordingly.
If there is a "pilot" shortage, its likely that 1) airlines can't find pilots of a certain rating or pilots who can fulfill some regulation of training or flight time or 2) the margins in the industry are so slim that the industry has a very narrow compensation window.
I suspect its the latter. Flying is commodity now. People fly like they used to take the bus 30+ years ago. If you're working the same economic niche as a bus driver, well, you will get paid like one.

I disagree with number 5. It is more skilled than ever. I mean you're right that we don't have to hand calculate our wind correction but working all these highly automated systems and troubleshooting them when they go wrong makes it one of the most technical jobs around. Much more technical than a fast food worker. I do agree that the shortage is more of people willing to fly for that wage who have the hours because you would have to be taking a huge pay cut from what got you there or else be drowning in student debt. The cost alone of the training should warrant a step above a living wage.

The only thing I would add to this is the management view of pilots as a negotiable commodity combined with the fact that airline union CBAs never expire, they become amendable.  With the status-quo wages locked in forever (until a new CBA is signed) there is little incentive for the company to negotiate in good faith to raise those wages -- pilot unions have no real access to self-help.  To strike you first need years of negotiation under the NLMB, a declaration of an impasse and then a 30 day cooling off period before you can strike (until the president orders you back to work)

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What happens when an airplane stalls and why do pilots practice it?
When a non-pilot hears the word stall, it brings to mind what happens when a car stalls -  the engine quits. It seems like that would be a dangerous scenario in an airplane.
From a non-pilot perspective, what happens when an airplane stalls and why is it important for pilots to practice it?

Stall was an unfortunate choice of words for an engine that suddenly quits since the aerodynamic stall in aviation means something very different and isn't related to the aircraft engine at all1.
To a non-pilot, an aerodynamic stall can best be described as the situation where there is not enough air flowing over the wings to create the amount of lift needed to hold up the airplane.
The main reason that student pilots practice stalls is to learn the telltale signs that occur just before it happens and to make the recovery procedure automatic.  If pilots can recognize an impending stall, they can take corrective action to either avoid the stall altogether or to recover as quickly as possible.
Outside of training, inadvertent stalls typically only occur shortly before landing and after takeoff, when the pilot gets distracted while already at a slow speed.  In both of these situations the airplane is very close to the ground, immediately requiring the correct action from the pilot in order to avoid a crash.  This needs to be instinctive and corrected using muscle memory so that it is accomplished as rapidly as possible.
The next logical question is usually:  How does a pilot fix an airplane that has stalled?
Fortunately, airplanes are designed so that even during a stall the tail is still effective2 and the pilot is able to use it to force the nose down.  This makes the airplane go faster, since it is pointed down towards the ground, and gets more air moving over the wing which allows it to create enough lift for the airplane to start flying again.  During practice it is usually pretty uneventful, but when it happens at a low altitude there may not be enough time to regain flying speed before the airplane crashes.
For more information, AOPA has a great Safety Publication targeting flight instructors called Why we teach slow flight and stalls which is available on their website.

1 However, the sail on a sailboat can also "stall" when there isn't enough wind and since they have been around since 3,000 BC I guess that technically this usage of the word applies to both situations.
2 There are some stalls in particular airplane designs known as deep stalls that can be unrecoverable.  I don't think that this is important when describing it to a layperson though.

You can also stall from overspeed (exceeding the critical mach number) and from flying too high.

@Aron True, but I'm not sure that it adds value when describing a stall to a non-pilot though.   :)

What's "uneventful" to one, is "terrifying" to another, and "awesome" to others.

@Aron I thought that you can stall from underspeed (well below the minimum required speed of flight) and from flying too high (and getting into the "coffin corner"). But overspeeding with a subsonic aircraft is associated with "crashing into the air", so to say,  and the aircraft breaking apart, but not with a stall. Am I wrong?

landroni: While your aircraft breaking apart may happen first with many subsonic craft, wings (and control surfaces) that are designed purely for subsonic flight can stall even if they do hold together in supersonic flight (or, more accurately, as Aron said, in flight above the critical Mach number.) The shock wave separates the airflow, potentially pushing it away from the wing/control surfaces. Wings and control surfaces aren't effective in a vacuum for obvious reasons.

An interesting addon to the second footnote is that this happens to (almost?) all fixed wing aircraft operated aft of their design cg envelopes. It's one of the easiest ways to see that you should pay respect to the weight and balance chart. Unfortunately skydiving operations tend to stage a demonstration every now and then, which is tragic but does provide an object lesson.

There's another reason to practice stalling: Landing an airplane is equivalent to stalling it -- slowing it down so it stops flying -- when the wheels are on the runway or the floats are a centimeter above the water.

@OllieJones Not to nitpick, but that's not always true. Large transport aircraft tend to be "flown onto the runway" and at no point should the airplane experience an imminent stall. Even small aircraft pilots often prefer to land slightly above stall speed to achieve a smooth landing, unless they are landing on a short runway and are using short-field technique. I would say "slow flight" practice is far more applicable to landing than stalls, because it teaches the student how to control the airplane in slow flight, as if on final approach.

"the pilot is able to use it to force the nose down. This makes the airplane go faster" -- a stall can happen in any attitude and at any airspeed. I've inadvertantly stalled a T6 upsidedown at the top of a loop because I didn't pull enough Gs to get over the top. Needless to say, my muscle memory didn't help, and I had to consciously pull the stick back to break the stall. I have also done a few accelerated stalls with the nose 20 degrees below the horizon by trying to pull out of the dive too hard during spin recovery

@rbp Well, you are still forcing the nose down, but in referebce to the relative wind instead of the horizon. Again, for the layperson that  this is targeted to, there isn't a need to explain the corner cases.  :)

good point, @Lnafziger, although "down" is based on your frame of reference

@reirab: What separates is flow above the wing and if there was vacuum below the separated flow, the wing would work extremely well. Alas below the separated flow is stagnant air at ambient pressure and that makes the wing not work.

@landroni: Air speed is highest over the wing, so the first effect of approaching speed of sound (actually occurs at about 70% of it with straight wings) is forming shock wave above the wing and associated loss of lift and shift of lift aft, which together cause the aircraft to pitch down ("mach tuck"). This pitch down causes the aircraft to accelerate further. Subsonic aircraft may not have sufficient control authority to pull out and eventually overspeeds to the point of structural failure or hitting the ground. It's why supersonic aircraft usually have all-moving elevator.

An engine stall and an aerodynamic stall are completely different. In aviation, an engine stall is referred to as an engine failure, and an aerodynamic stall is simply referred to as a stall.
For Nerds
An aerodynamic stall happens when the wing stops producing lift because the Angle of Attack is too high. This is usually, but not always, caused by pulling back on the stick without adjusting power appropriately. Various factors including the weight of the aircraft, flaps, and icing can change the angle of attack the aircraft stalls at.
For the rest of us
A stall happens when the wing no longer creates lift. This happens when the speed of the air going over the wing decreases too much. Basically, this is why the advice one mother gave her son during World War I, 'Fly low and slow, I don't want you to get hurt!' makes very little sense aerodynamically speaking.
Why pilots practice stalls
Pilots practice stalls to learn the warning signs that they are entering a stall, and to practice recovering from a stall should they ever end up in one.
Also, as brinky stated in a comment to this answer:

While in flight training or when moving to a different airframe,
  pilots practice putting the aircraft in a stall and then recovering
  from it to also develop "muscle-type" memory, because stalls are
  probably the single most unsettling and unpredictable behavior that
  all aircraft share, and different airframes may react very differently
  to stalls. Stalls are unforgiving and must be rectified promptly, due
  to rapid altitude loss.

Good, but one more point which is the same that I mentioned to p1l0t on his answer: A minor point that you may want to consider editing: When a wing stalls it is still generating lift (it doesn't come plummeting out of the air at terminal velocity), it just doesn't produce enough lift for the airplane to maintain altitude.

@Lnafziger: ah, ok, will add that

I wouldn't say a stall is merely when the wings are not producing enough lift to maintain altitude, though. Any descent requires the wings to produce less lift than what is required to maintain altitude. Aerodynamic stall is entered at the point where smooth airflow separation from the wing has reached the point that further increase in angle-of-attack will produce less lift rather than more. A normal descent is very different and still has smooth airflow over the wings with little or no separation.

Ah, that's right

@reirab Well, I didn't say that any time that there isn't enough lift to maintain altitude that it is a stall.  Just that when an airplane has stalled that there isn't enough lift to maintain altitude.  (Or rather, I am pointing out that when he says that the wing "no longer creates lift" that it is technically incorrect since it will still be producing some lift, even when it's stalled.)

@reirab: To further add to the confusion, not all descents require the wings to produce less lift than necessary to maintain altitude. A constant rate of descent actually requires roughly the same lift as maintaining altitude.

@MarcksThomas - Good point. Obviously, the total upward and downward forces have to be balanced to maintain a constant vertical speed. The total lift is dropped below normal in order to enter the descent, but is then increased back to normal once the desired rate of descent is reached in order to maintain that rate.

The stalls that pilots practice are aerodynamic stalls not engine stalls. It happens when the critical angle of attack is exceeded. [Typically the nose is pitched up too much is what it means]. It results in airflow separation that means that the wing no longer is generating any [significant] lift. As soon as the pilot recovers by letting the nose down and regaining some airspeed they are flying again. We practice these for the purpose of being able to recognize the onset of a stall and being able to recover from an inadvertent stall. This is important because while it is actually kind of fun at altitude, near the ground (like while on approach or departure for example) it can be deadly.

P.S. there is also something called accelerated stalls but it's a bit more complicated.

Pilots practice engine failures too :)

SIMULATED engine failures we don't actually shut it off. At least not in a single-engine.

@p1l0t and roe - I used to teach full power-off engine failures and restart procedures. But I always made sure that I was at altitude and I was close enough to get back to the airport in case I also ended up unintentionally demonstrating power-off landings. :-|

I don't see the point in that, the difference with having the motor at idle is minimal but it does provide you with a second chance. Especially with a student at the controls. You're being sarcastic I hope.

It was more for the restart procedures. I've taught full engine-off procedures in singles and twins. I always felt it was good to demonstrate a full feathering in certain twins. It's not always as easy as it should be and zero-thrust does't exactly match the performance of a completely shut down engine, especially one with an unfeathered prop. And seeing how much distraction is involved in just restarting an engine makes you appreciate having a second pilot aboard to handle the extra load. That said...

That said, I do agree with you completely. The older I get, the more I understand the saying "There are old pilots and bold pilots, but no old, bold pilots." I taught it because that was how I was taught. In hindsight, I don't really know how much of a benefit was gained with shutting any engine completely off. I was ready if it ever happened to me. And my students were prepared. But considering the risks, I don't know that it was worth it. But that's also why I never strayed far from my airport when doing those. :-|

I have shut one down in a twin but never in a single. If for some reason you can't get it restarted, even if you do make it back without a hitch the FAA is going to want a report if they find out. I fly out of a towered airport if I pulled a stunt like that.. I mean they got on my case because I aborted a takeoff without warning them first. A good pilot should have the skills but I just think of all the bad scenarios, the key falls under the seat, you make an announcement on the radio for a short approach, FSDO calls the next morning.. or worse you end up in a field.

@Shawn My first CFI did a full engine shutdown on my discovery flight. Once the initial panic subsided, it was fun!  Only later did I realize how stupid it was. He eventually got fired for pulling the same stunt with the school owner’s son, who reportedly wet himself.

@StephenS LOL I'm only laughing because no one got hurt. I'm glad they canned that guy though. A windmilling propeller is pretty easy to restart, but just too much risk in a single-engine even though the idea is to train for that [mostly] survivable scenario.

When the airplane is at a "regular" angle of attack (angle with the direction of the wind), with the nose more or less forward, the wing works as designed and produces lift.
If the airplane turns the nose straight up while continuing to go forward, it's intuitive that the wings stop producing lift, as they are just vertical walls against the wind at this point. (The plane can still fly in this condition if the engines alone can pull all the weight, but that's only common in fighter and acrobat planes).
It turns out that the transition between these two is relatively sudden. As the angle of attack increases, wing lift goes up and up and up, then suddenly drops sharply as the smooth air flow detaches from the back of the wing. That's the stall. It can also happen when lowering speed while keeping the angle constant.
As to why pilots practice it, nothing to add to what flyingfisch said.

Stall has to do with the attachment of the boundary layer on the upper surface of the wing. When the airflow separates (video) from the surface it stops generating lift and the wing stalls. Lift is the aerodynamic force that keeps the aircraft airborne. It's the reaction of the air to the mass of the airplane.
Now, lightheartedly, to make a super simple analogy, let's say that the equivalent of lift when you walk is the upward reaction of the floor to the mass of your body (which is actually true); imagine also that you are walking and suddenly a trapdoor opens under your feet (airflow separation). As soon as you feel that you're falling you instinctively open your arms trying to grab something while your eyes are looking around for something to hold on, say a handle or the edges of the trapdoor. This is the situation that one could say that you have stalled. Your normal walking was interrupted and now you're falling.
Pilots train in order to recognize the symptoms and learn where the "handles" are -i.e. necessary actions to take- when this situation occurs.

"Stall" in aviation generally means an aerodynamic stall. An "engine stall" would be called just that.
There is an excellent understandable writeup of an Airbus crash from a stall, Air France flight 447, on the UK's Daily Telegraph.
A wing provides lift to a plane as a direct result of the air flowing over the surface. A stationary plane falls - without airspeed, the wings cannot provide lift. A stall occurs when the airspeed falls too low, and the lift provided by the wings cannot maintain flight.
Airspeed can fall too low for several reasons; for AF447 it was the most common case, that the plane's angle of attack was raised too high, and the engines could not provide enough thrust to keep the plane flying above its stall speed. In this scenario, memory of stall practices should kick in and the pilot should point the nose down to regain airspeed, allowing the wings to provide more lift.
Interestingly, the article suggests that the Airbus controls take commands rather than being a modifiable reflection of the current state of the aircraft, so once the stick was released it wasn't apparent that the plane was trying to climb to the other pilot. Eventually airspeed dropped so low that the airspeed sensors themselves switched off, silencing the stall warning. When the pilots realised the mistake and pointed the nose back down, the increasing airspeed re-enabled the airspeed sensors, which restarted the stall warning, perhaps confusing the crew further. The captain realised just before impact: "10 degrees of pitch..."
Practising of scenarios is to prepare crew so that in the event of some danger or failure, they do not panic but respond appropriately and correct the problem.

A stall can be observed visually on an aircraft with a flexible cloth wing, such as a hang glider or micro-light. To land a hang glider, one deliberately triggers a stall which spreads out across the wing surface, and you can see the fabric going limp.

In paragliding, the pilot stalls the wing on purpose sometimes in order to recover the glider from some situation - when the lines are messed up or there's something else.
It is similar to rebooting your computer.

A stall is when the plane exceeds the linear and predictable. If it hits the air too steep, the flow over the wings collapses into chaos and the behaviour is not always predictable.
Pilots practice to develop the skills to recover into the linear range of flight
See also spins and look up the number of people who have died after stalling a "laminar flow" wing (the Gee-Bee racer comes to mind here).

A stall is when there is not enough air flowing over the wings to achieve the amount of lift necessary to keep the airplane flying. The telltale sign of an impending stall is when the airplane begins to buffet. The recovery procedure includes unloading the airframe by pushing forward on the yoke(the airplane equivalent of a steering wheel) and adding engine power. In a stall where the airplane's weight and balance is well configured, the nose will be heavier and will come down once the wings have stalled, making it easier for the pilot to recover even if the tailplane is not super effective after the stall. Unrecoverable stalls result when there is an aft Center of Gravity, and there is not enough authority in the elevator to force the nose down.

While the information is correct, this does not attempt to answer the question of why pilots practice it?


text:
How do I contact Flight Service while in flight?
How do I contact a Flight Service Station (FSS) in the United States in order to receive weather information or open a flight plan after I've already departed.

You can also open flight plans with the tower or local approach control.

@Dave, you cannot "open" or "file" VFR flight plans with ATC. You can only do that with FSS. You can request flight following with ATC, but we have zero interaction with the "VFR flight plan" system.

For filing and opening VFR flight plans, pushing back your ETA because of a delay, and a few more obscure things which you can find in section 4-1-21 of the AIM you would contact Flight Service (addressed as "region Radio" - e.g. "New York Radio" or "Bridgeport Radio")
The universal flight service frequency is 122.2, and above 5000 feet you should be able to get someone on that frequency, but it's not guaranteed.
If yo can't raise someone on 122.2, or you're below 5000 feet, there are local Flight Service frequencies are listed on VORs as shown below. As a general rule if you can receive the VOR you can talk to Flight Service with the frequencies listed, and using these local frequencies helps avoid congestion on the universal frequency.
(skyvector.com)
Initial calls to Flight Service should take the form:
<Facility Name> Radio, Cessna 12345, 5 northeast of Hampton VOR
(as above for Flight Watch, Who you're talking to ("New York Radio" in the example above), Who you are, and Where you are.)
There's a little catch with Flight Service frequencies: If there is an R after the frequency (as shown for 122.1 here) it is a receive-only frequency. You will talk to flight service on that frequency, but you will listen to the VOR's audio channel to hear Flight Service's responses.
When you call Flight Service using one of these frequencies make sure your nav radio is tuned to the VOR and the voice channel volume is up, and let them know which VOR you're listening to as opposed to your location.
In this example if you were contacting Flight Service on 1221 the call would be New York Radio, Cessna 12345, receiving on Hampton VOR.

For weather (giving PIREPs or asking for a weather report) you will also talk to Flight Service: There used to be a separate service (Flight Watch) and frequency for this, but weather-related tasks were consolidated into the general Flight Service frequency in late 2015.
Radio coverage and contact procedures are identical to what was mentioned above for dealing with flight plan stuff, but you should make it clear you're looking for weather updates (add "requesting enroute weather") or that you want to give a PIREP (add "With a pilot report") so the Flight Service Specialist on the other end of the radio knows what you're looking for.
As weather is generally a localized phenomenon (and weather updates or PIREPs can take some time to relay) it is probably be a good idea to use the local flight service frequencies for weather-relatedrather than the universal 122.2 frequency to avoid congestion there, though I don't believe the FAA or Flight Service has issued any official guidance on this since Flight Watch was consolidated with the rest of Flight Service.

Are the Flight Service frequencies available 24 hours a day?

@Magnetoz The Flight Service frequencies (122.2 and the ones published on VORs) are 24/7. There are some "part time flight service stations" but their frequencies are supposed to be switched to a 24-hour station when the local station is closed. (If that doesn't happen or you don't get an answer you can always try on 122.2 - Worst case Flight Service will sometimes ask you to switch to a discrete frequency if 122.2 is congested.)

Just an update: Flight Watch was absorbed into Flight Services this month. I think the 122.0 frequency still exists.

@EdwardFalk Yup, I forgot I wrote this answer - I guess I should edit it :-)

@EdwardFalk 122.0 is being phased out effective Oct 1 2015. If you call up on that frequency, Flight Service (for now) will advise you of such. In the next several months, it will become unmonitored. 122.2 will still be available for all services.

OK, so the initial call would be e.g. "New York Radio, N1234A near HAMPTON on 122.6, over", correct?

You get the same facility regardless of the frequency. eg, if you contact "New York Radio" and cite "Hampton VOR" as your position, then New York Radio will answer using the antennas they have at Hampton, regardless of the frequency.
Regarding congestion, you may get less frequency congestion, but you will still have to contend with the same amount of facility congestion. Meaning, if there are 4 aircraft calling, 2 on the VOR frequency and 2 on 122.2, then they will still service you in the order in which you called up.


text:
About how much drag does a non-operating engine create?
I think the drag of the engine is high, for most type of engine, e.g turbo fan, jet, ramjet... I guess the engine's drag is worse than a flat plate of the same area perpendicular to air stream because the engine slows the air and compress it, and also the engine has many components inside it that will interact with air.
A flat plate is very bad aerodynamically and so the engine. Even small landing wheel should be folded inside body for better drag. So an aircraft with engines turned off will have very high drag, but I can not find any estimation on this matter.
For example, an aircraft that has L/D about 15, how about its drag if engine is removed, e.g. for better glide ratio when engine failure?
Can you estimate this drag?

I don't know much about aerodynamics, but see if you agree with this simplification.  The non-operating engine lets some air through while the flat plate lets no air through, therefore in general the flat plate creates more drag.  I think it is not important what happens to the air inside the engine.

An inoperative engine creates much less drag than a flat plate of the same cross section. According to Sighard Hoerner's Fluid Dynamic Drag, the drag coefficient of a flat plate is 1.17. An engine nacelle has rounded intake lips which help the flow to stay attached while flowing around the nacelle. The closest of the generic bodies in the table below would be the sphere (drag coefficient of 0.47).

Figure 33 from Sighard Hoerner's Fluid Dynamic Drag, Chapter 3. Left column: Bodies of rotation; right column: Cross sections of 2D-bodies.
Much depends on the detail of flow separation at the forward corner, and here modern engines are rather good. If the flow stays attached, drag will be much lower than with the massive separation around and behind the flat plate. Air flowing out from the inside and over the corner of the flat plate will need some space to "turn around", effectively increasing the blocked cross section that the outside flow experiences.
Note that the reference area for all values here is the cross section of the body perpendicular to the flow, while the reference area for airplanes is their wing area.
Depending on the type, the diameter of one GE90 engine is 134 or 135 inches. Two of those power a Boeing 777 which has a wing area of 427.8 m². The ratio of engine frontal area to wing area is thus 1/23.335. The (estimated) drag coefficient of the nacelle of 0.5 would shrink to 0.0214 when referenced to the wing area. This would increase the total drag by about 50%, so instead of an L/D of 18 without engines, the airplane would have an L/D of 12 with inoperative engines.

I know that the case of engine is smooth, but IMO you don't analyse the stream that run through the core. If the engine is turned off, that stream will rotate the blades of the turbine, and while the core is rotated, it will compress air, which may consume much more energy than a flat plate

@user2174870: no, the core will rotate slowly and with separated flow on the blades. No significant compression will happen, and the flow rate is insignificant. Only the fan might spin a little faster, but again without compressing anything and much less flow rate than in operation. As a first approximation, you may assume that the engine blocks the air from passing through it. With propellers, this is slightly different, because a non-feathered propeller will crank the engine, which will indeed compress the air in the cylinders. A spinning propeller will cause more drag than a stopped one.

When an imperative engine is being shuttled by an aircraft do they plug the intake or lock down the fan or anything?

@TomMcW: They are just hung under the wing, and as far as I know they do not need to be locked down. See here for the SE question on the topic (which does not go down to those details, however).

There has been some studies carried out for calculating the windmilling drag (of turbojet) engines and these indicate that the windmilling drag is in the order of 10% of the net thrust. In modern high-bypass turbofan engines, the figure will probably be higher.
NASA simulations estimate the baseline drag coefficient of engine for a medium sized aircraft as 0.31, with additional drag based on Mach number.

Drag due to windmilling, Image from Validation of an Integrated Airframe and Turbofan Engine Simulation for Evaluation of Propulsion Control Modes by Jonathan Litt et. al.
There are a number of studies that deal with the windmilling characteristics of turbojet engines. In general, the effect of removal engine drag in the baseline L/D will not be very high in all probability.
You're also correct that the drag is more in case of windmilling compared to locked rotor. The drag in that case is in the order of 40% of the windmilling engine.
However, there are multiple problems with the premise that the engines would've to be dropped in case of engine failure.

Engine failure is pretty rare for a system like engine jettison to be incorporated. In addition to the weight increase, there is the problem uncommanded operation.
Aircrafts are certified for OEI condition. Nobody is going to drop a costly engine (note that multiple engine failures are extremely rare) because it stopped due to a minor problem. Better to fix it once the aircraft touches down.
Even after all this , where are you going to drop the engine? unless it is over the ocean, there is no certainty that the falling engine will not cause any damage.
Assuming you drop the engine and it doesn't damage the aircraft, you've to trim the aircraft to adjust for the new c.g.  Also, aircraft will be unbalanced if you drop one of the engines, requiring more control force. Also, the wing twist will increase and the wing will bend up.

What is a drag coefficient worth without the reference area? This answer is incomplete.

Also, the engine helps to relieve the bending moment of the wing, dropping it will increase the loading on the wing.

@aeroalias, you mention the value of 0.31, but I don't see that value in the graph? Is it mistake, or is it referring to another case? Some clarification would be helpful :)

@ROIMaison 0.31 is the baseline value. The graph shows the additional value due to Mach number. Note that the graph begins at Mach 0.2, which is about the takeoff speed for the (simulated) aircraft. So, for eg, at Mach 0.4, the value will be 0.31+0.185 = 0.495

Ah yes, now I understand. Thanks

@PeterKämpf, according to the report the reference area is the inlet area of the engine

@ROIMaison: Thank you! Yes, I did not read the report, and using the inlet area is one way. I used the full cross section area in my answer and get much the same drag coefficient from Hoerner, so the actual drag is lower than what I estimated. Makes sense when the shape of the nacelle is more elongated than a simple sphere.

On the contrary I think dropping engines is a great idea. Hours of fun there deciding the priority list and then working out the most efficient and productive "drop" based on your location at the time. To get started could I suggest the UK the chancellor, prime minister, leader of the opposition, my ex wife, and my next door neighbours dog? Of course it could be pilot specific such as his/her ex partner, or his/her airline CEO, mum in law, etc. etc. I know the list could be long but that's what we have computers for right?

One method is having a party so all of them can gather at one place, right? Or you can design a plane that have multiple engines, 10 or 15 engines depend on your list's length.


text:
Which way should you turn to avoid another aircraft?
During visual flight conditions, when you see another aircraft in your path, you should strive to avoid hitting it.
In shipping there are standard international rules about which way boats should turn in order to avoid each other.
Are there similar rules in aviation?

In shipping there are standard international rules about which way boats should dodge each other.
  Are there similar rules in aviation?

Why yes, there are - in fact they're basically similar to the maritime rules!
They're described in ICAO Annex 2 (or for US pilots, FAR 91.113).
Basically, for aircraft of the same type (e.g. two airplanes) in the air the rules are:

Approaching head-on: Both aircraft alter their heading to the right.
Converging: Give way to the aircraft on your right.
Overtaking: Pass "well clear" to the right of the aircraft being overtaken.
(The aircraft being overtaken has the right of way. The overtaking plane maneuvers to remain clear.)

The rules for converging aircraft are more complex when the aircraft are not the same type - basically the more maneuverable aircraft gives way to the less maneuverable aircraft (because something like a balloon can't really maneuver to avoid a collision) - you can read the details in either of the links above.
Also while it's not explicitly spelled out in ICAO Annex 2 (or at least I didn't see it) aircraft in distress generally trump the "right of way" rules. (This is really common sense: if an aircraft is recognizably in distress you do everything in your power to stay out of their way and help ensure a safe outcome to whatever their situation is.)

Also, powered aircraft give way to non-powered aircraft, regardless of maneuverability, so in theory a B737 should turn to avoid a glider...

Are those rules( ICAO ) acknowledged internationally; would they hold up in most countries?

@self. You would have to check the specific regulations in each country, as those define the operating rules within that nation's airspace (ICAO signatory states are allowed to deviate from ICAO standards, and there are some non-ICAO states), but these rules hold in all the countries I'm aware of.

@voretaq7 Thanks.

This is not answering your question directly but is relevant IMHO.
There are also rules of thumb designed to prevent aircraft from arriving at the same place at the same altitude heading in opposite directions for example:

Follow line features (roads, rivers etc) by placing them on your left.
The quadrantal rule for altitude selection (optional for VFR, but so is common sense). http://en.wikipedia.org/wiki/Flight_level#Quadrantal_rule
Not arriving at common waypoints at "obvious" altitudes.  Use 1100 instead of 1000, 1450 instead of 1500 etc.

If many pilots followed number 3, then it would defeat the purpose, wouldn't it?

@deletethisaccount I think the point is that you should not be at 1000ft or 1500ft, not that you should specifically choose 1100 and 1450 as alternatives.

Of course if everyone followed #3 the safest altitude would be the "obvious" one.  Just to be clear, #3 may be common practice but it is NOT a rule.  The only rule approaching a waypoint is be alert!

@DavidRicherby  Yep, that's the point.  It's a bit overkill but can be useful.  Where I trained, there is a particular waypoint (CPT in the UK) closeby which seems to be on every training route for x-country solos etc in the south of England. There were usually 3 or 4 craft all within a few miles of the WPT at any time and picking a random "weird" altitude does give some increased separation.

#1 is good common sense in the US (not sure re/ the UK), but I've never seen it mentioned in the regs or the AIM.

The comments are valid, leading from my bad choice of the word "rules".  Now edited.

The old Quadrantal rule of the UK has now been replaced with the hemispherical rule which I understand is used by most of the world's regulatory bodies. And under the FAA, at least, a version of that rule does apply to VFR cruising flight above 3000' AGL.


text:
Can the Airbus A380 safely fly with two engines out on the same wing?
I know twin-engine jets can safely take off and fly with one engine out (i.e. all thrust only on one side).  Of course quad-engine jets are also able to fly safely in case one engine is out.
But is the A380 designed to also fly safely in case two engines on the same wing are out?
Would the rudder be able to compensate the thrust imbalance?
I'm referring to an engine-out situation in flight, not during take off.  I would assume that the problems aggravate during take off since the rudder has less airflow to counter the imbalance.
Note: Two engines out on the same wing is not a very likely scenario, but it's not impossible.  For example, on Qantas 32, damage from the number 2 engine explosion impacted fuel and control lines of engine 1.

Just as an aside, I don't believe that the A380 is certified to takeoff with two engines out, regardless of which wing they're on. So it's slightly more than "aggravated" during takeoff :) good question regarding in-flight, though

This article is relevant.

I imagine the answer to this question would be the same if you replaced "Airbus A380" in the question with any other passenger airliner with four jet engines.

I don't think any twin jet can safely take off with one engine out.  Vrotate would not be achieved soon enough and would necessarily require aborting takeoff.  The tough decision would be if an engine failed just after rotation, what is the best course of action:  land immediately (possibly rolling off the runway end), or try to go round.

@wallyk -- in your failure-at-rotation case, since you're above V1, you continue the takeoff.  The only cases where you wouldn't is if your engine didn't just quit, but turned into a big drag bucket that's keeping you from maintaining flying speed (certain propeller failures on turboprops can do this, and so can reverser deployment on a jet; it's also possible to get this with gross engine damage.)

If a third engine went, would the final one provide enough thrust to maintain level flight?

@trengot You'd be in a slow driftdown in that case -- cascading engine failures in multi-engine aircraft are exceedingly rare though, the only instance I have heard of was with a Kalitta Air B742 that had multiple marginal engines on it, so when power was applied after the first engine failure, the remaining engines went as well, one by one.  (It's actually more common for all engines to fail at once due to issues like fuel exhaustion, volcanic ash, etal.)

@wallyk All twin jet passenger airliners are certified to continue a takeoff and climb-out if an engine fails after V1. If an engine fails before V1, they'll abort the takeoff, but they'll continue if it fails after V1. This happens occasionally in practice, such as in bird-strike events. For example, here's a 757 continuing with an engine failure after V1 and here's an A330 aborting during a failure before V1.

I routinely train pilots in the 380 sim to make 2 engine-out approaches; the aeroplane behaves very well....so long as you understand the aerodynamics involved.

@UnrecognizedFallingObject: I've found another example: in 2013, the #2 engine came off the wing of a modified 707 and knocked the inlet cowling off the #1 engine in the process.  This caused the #1 engine, although still running, to not produce enough thrust to counter the drag it caused; with (effectively) two engines out,  the plane was unable to maintain flight, and the captain made a forced landing on what was left of the runway.  Everyone made it out OK, although the plane was destroyed.  Link here.

@Sean -- yeah, I'm aware of the N707AR mishap -- that was a case where the #1 engine was effectively producing negative thrust, which put the asymmetric thrust outside the certification limits.

@wallyk twins have enough spare power to take off on 1 engine.

Proving that a multi-engine aircraft is flyable with one or more engines inoperative is part of the certification programme. The Airbus A380 is certified under FAR (Federal Aviation Regulations, USA) part 25 and JAR (Joint Aviation Regulations, Europe) part 25. One of the requirements of FAR/JAR 25 is that the directional control can be maintained when two critical engines (that means under the same wing) fail.
The following is selectively extracted from FAR part 25 (JAR is similar, there are some differences). Emphasis added by me.

Controllability and Maneuverability
§25.143   General.
....
b) It must be possible to make a smooth transition from one flight
  condition to any other flight condition without exceptional piloting
  skill, alertness, or strength, and without danger of exceeding the
  airplane limit-load factor under any probable operating conditions,
  including—

The sudden failure of the critical engine;
For airplanes with three or more engines, the sudden failure of
  the second critical engine when the airplane is in the en route,
  approach, or landing configuration and is trimmed with the critical
  engine inoperative;

...

$ $

§25.147   Directional and lateral control.
a)  Directional control; general. It must be possible, with the wings
  level, to yaw into the operative engine and to safely make a
  reasonably sudden change in heading of up to 15 degrees in the
  direction of the critical inoperative engine. This must be shown at
  1.3 VSR1 for heading changes up to 15 degrees (except that the heading change at which the rudder pedal force is 150 pounds need not be
  exceeded), and with—

The critical engine inoperative and its propeller in the minimum
  drag position;
The power required for level flight at 1.3 VSR1, but not more than
  maximum continuous power;
The most unfavorable center of gravity;
Landing gear retracted;
Flaps in the approach position; and
Maximum landing weight.

b) Directional control; airplanes with four or more engines.
  Airplanes with four or more engines must meet the requirements of
  paragraph (a) of this section except that—

The two critical engines must be inoperative with their propellers
  (if applicable) in the minimum drag position;
[Reserved]
The flaps must be in the most favorable climb position.

c) ...
...
d) ...
e) Lateral control; airplanes with four or more engines. Airplanes
  with four or more engines must be able to make 20° banked turns, with
  and against the inoperative engines, from steady flight at a speed
  equal to 1.3 VSR1, with maximum continuous power, and with the
  airplane in the configuration prescribed by paragraph (b) of this
  section.
f) ...

$ $

§25.149   Minimum control speed.
...
g) For airplanes with three or more engines, VMCL-2, the minimum control speed during approach and landing with one critical engine
  inoperative, is the calibrated airspeed at which, when a second
  critical engine is suddenly made inoperative, it is possible to
  maintain control of the airplane with both engines still inoperative,
  and maintain straight flight with an angle of bank of not more than 5
  degrees. VMCL-2 must be established with—

...
...

$ $

§25.161   Trim.
...
e) Airplanes with four or more engines. Each airplane with four or
  more engines must also maintain trim in rectilinear flight with the
  most unfavorable center of gravity and at the climb speed,
  configuration, and power required by §25.123(a) for the purpose of
  establishing the en route flight paths with two engines inoperative.

A long answer, but very informative.  Could you describe briefly (or link to such description) what JAR and FAR are?

JAR and FAR are the aviation regulations of Europe and the USA. Some parts address design of aircraft, other parts address operations, licensing etc. The JARs are gradually being replaced by EASA (European Aviation Safety Agency) specifications.

So they only require that the aircraft remain controllable if the crew has time to compensate for the first engine failure before the second one fails - meaning that two engines on the same side going down simultaneously could potentially result in a loss of control?

Would the rudder be able to compensate the thrust imbalance?

It depends.
The best way to translate those regulations into practical examples is to look at the certification of the A330 and A340-300, which have approximately the same amount of total thrust, the difference being the engine configuration.
Takeoff performance requirements are predicated on 1-engine out for both configurations. Hence you'll notice that the minimum climb gradient requirement means that the 4-engine loses less thrust, hence is capable of a higher takeoff weight; around   30-40T more.
Two engine out comes into play during the landing config, where VMCL2, two engine out minimum control speed in the landing config, is much higher than VMCL1; two engine out 340 has approximately the same amount of thrust as a one engine out 330, but the thrust line produces a greater moment on the 340. Incidentally these speeds don't change with weight and depend on handling qualities and directional control at the most unfavorable CG position, which is the AFT limit. VMCL2 is around 160 knots, which is well above normal landing speed, so at some point during the approach when landing is assured, the pilot will elect to reduce below the VMCL2 to normal landing speed. It will not be possible to select full thrust without losing directional control in this regime of flight, however the benefits of landing at lower speed offset this.
Back to the takeoff case with two engines inoperative, although VMCL is not specifically defined for takeoff config and regulations don't specifically cover this scenario, at higher takeoff weights minimum control speed becomes less of an issue compared to VS1G and overall climb performance. While at lighter weights, it maybe possible to reduce thrust to maintain control if performance permits.


text:
A320 procedures for one thrust reverser inoperative; why is the old procedure more efficient?
I recently watched the Mayday series about TAM Airline Flight 3054. Landing on a short runway with only one thrust reverser in use, and the documentary explained something like:

The old Airbus procedure was to put both engines' controls on IDLE and then put only one of them on REVERSE. The new procedure says that you put both of them on REVERSE, regardless of if one of them doesn't work. The captain seemed to aim at the old procedure, because it is known to be more efficient.

I wonder how it is more efficient? Does the position of the controls for the second motor really influence the behaviour of the first one?

Does the position of the controls for the second motor really influence the behaviour of the first one?

No, it influences the activation of various other systems like the autothrust, the airbrakes, as well as the thrust reversers.

how it is more efficient?

Because in the new procedure the engine with non-functioning reverser starts to spool up before the system realizes the reverser did not deploy and cuts it again, so for a moment it produces forward thrust. The old procedure avoids this.

So if you put the engine control at REVERSE and the reverser is not working, you produce a forward thrust?

@tohecz: Correct. Reverse is a throttle setting above idle.

Ok, that explains it all. Thanks a lot!

@RedGrittyBrick: Reverse in Airbus is a throttle setting below idle. If the new procedure is to pull both in reverse, the FADEC is clever enough to leave the engine at idle if the reverser did not deploy, otherwise they couldn't make it that. That's in contrast to other aircraft like Tu204 where the reverse is separate control and thrust levers are advanced and reversers failing to deploy caused two incidents and an accident in quick succession last winter.

@JanHudec So you suggest that the true explanation is different?

@tohecz: Yes, I suggest the part about being more efficient needs different explanation. (Note: in the accident the engine did produce forward thrust, but because the procedure was not followed. Neither procedure.)

@JanHudec However, it says that "captain probably decided for the old procedure since it was better, and failed to follow it."

Page 73 of the final report says basically what this answer does. When throttles are put in reverse, the engines automatically spool up. If a reverser doesn't deploy, that engine goes back to idle, but the initial spool up is what makes braking less efficient.

@fooot: Thanks. Without saying that the answer as written is misleading. I'll propose better wording.


text:
Where can I find numeric maps of aerodromes?
I desperately need a map of an airport aerodrome like the ones used for tracking the movements of aircraft similar to the picture below and I don't know where I can find these ones :

You should try to explain what you plan on using this map for, then it will be more clear to people exactly what you need.

Airport Diagrams such as this one of KPNE

are published here on the FAA website as well as in in the airport facilities directory. These are up to date and considered legal for navigation as far as I know. If you are looking for them in another type of format that may be tough, you can try reaching out to the specific airports or the FAA directly.

These are also in the Canadian flight supplements. :)

Well I asked the airport for it and they don't have one just the pdf format !

OpenStreetMap is a good place to start.
I've used it for a project with real aircraft data and found that the OSM map of the airport I was interested in had a small offset. Using JOSM I was able to correct for the offset and add missing taxiways and additional features.
I have also tried to use PDF based airport diagrams from various Aeronautical Information Publications (AIP) but I was not successful in extracting the specific elements I was interested in.

I saw the osm map and it seems correct not needing modifications I just have to change the colors of the map and it looks difficult!

Can you explain what you are trying to achieve, what are you building?

I wanna simulate the traffic of aircraft on the aerodrome that's why I need the map so I can project the airplanes on .

I think OSM support map styles in some form of CSS. Perhaps that would help you. Note that airport surfaces have key value pairs in their XML definition like this: <tag k="aeroway" v="apron"/>

okey! I'll check that thank you.


text:
What happens after a rudder hardover?
I'm sure many of you are familiar with "rudder hardover", where an aeroplane's rudder deflects to an extreme position - full left or full right. Many of you will be aware that such an event, caused by a faulty Power Control Unit, was implicated in the loss of United Airlines Flight 585 and USAir Flight 427. A rudder hardover was also involved in an incident on Eastwind Airlines Flight 517 and the investigation into that incident unravelled the mysteries around the loss of the two aforementioned flights.
My question is about the effects of a rudder hardover on flight - and how it led to the two crashes. I'm interested in what happens, aerodynamically, after the rudder has moved to its extreme position and the sequence of events which follows. It's obvious to me that a rudder hardover must be an extremely destabilising event. But I'd like a more detailed explanation of precisely what happens. What effects does it cause? What happens to the affected aeroplane and why might it crash?

With a fully deflected rudder the airplane will fly with substantial sideslip, and by dropping the windward wing it can be held on a straight course. This is usually done intentionally with gliders and light aircraft when they want to lose altitude quickly, because the high sideslip angle will increase drag substantially. If $\zeta$ is your rudder deflection angle, the slip will normally have a sideslip angle $\beta$ of between -$0.5\cdot\zeta$ and -$\zeta$, depending on the details of the aircraft.
If the hardover happens at high speed, the resulting aerodynamic forces will certainly overstress the fuselage and tail structure, and the aircraft will break apart and crash. No amount of pilot skill can prevent this!
If the aircraft survives the hardover incident, the best the pilot can hope for is a controlled crash: The sideslip angle will cause the landing gear to be torn off, and the dropped windward wing will provoke a ground loop - if the pilot tries to level the wings during the flare, the resulting yawing motion will ground loop the plane in the other direction.
I once had the chance to fly the prototype of the Swift S-1, an excellent Polish aerobatic glider. During one maneuver my left pedal got stuck, and I saw myself already sideslipping the plane into the ground. In the remaining minutes I tried to get the pedals unstuck and got lucky, ending the flight with an uneventful landing. Afterwards, I found out that the pedal mechanism had caught the wire from the battery, and my maneuvering had freed it up again. This was the closest I ever came to land with a rudder hardover, and I am not eager to ever try this out for real.

I'm glad you survived the close call. You're much too valuable to this coming.

Ps, your answer is based around unpowered flight. How would thrust steering affect the navigability equation?

Agreed, mostly -- it takes quite a bit of work to land a jet safely with severe crosscontrol (the EGF4649 crew was able to do it with lots of aileron, and heavy tiller through the rollout).  However, re: AA587, that required alternating rudder deflection to happen -- a simple full-deflection command is structurally fine at any speed below V-sub-a.

@RoboKaren: Thrust would help a little in level flight by the yawing moment of asymmetric thrust, and again during landing by reducing the sink rate. The yawing moment due to thrust depends on engine location, but is generally small - remember, the tail is sized to compensate for failed engines on one side.

@UnrecognizedFallingObject: Correct, but $v_C$ >> $v_A$ in most cases. When the hardover strikes during cruise, structural failure is very likely. The plane's reaction will not just be a simple yaw, but a yaw, overshoot, oscillation (hopefully damped), and normally the first overshoot is what gets you.

With regards to American Airlines Flight 587, is it more correct to say a rudder hardover caused it to break apart or the repeat deflection of the rudder surface to a much greater degree than necessary? I mean technically is hardover not the "jamming" of the control surface to a specific deflection due to mechanical failure as opposed to pilot input? Can we refer to both full deflection due to pilot input and full deflection due to hydraulic failure as a 'hardover event'?

@Ksery: It was the repeated deflection into the wind which tore the vertical off American Airlines 587. The vertical is sized for full deflection at zero sideslip and v$_A$, but not the added load of a sideslipping aircraft. And a commanded deflection is not a hardover in my book.

Hardover of a control surface basically means that it has become jammed against a mechanical stop; think of having a poltergeist in your plane, applying full rudder (or aileron, or elevator) one way or another.
In the case of the rudder, it indeed will cause a severe yaw and roll with the resulting sideslip.  Dealing with this requires a hefty helping of opposing aileron control (and spoileron, if your plane has that); this creates a forward slip, which keeps the plane on course at the cost of increased sink, which requires more pitch and power to overcome.
For most jets, this is hard on the crew, but doable: American Eagle 4649's crew shot an ILS into Bangor in blowing snow with their Embraer 135's rudder jammed full right, landing safely.  However, in the 737's case, the rudder has so much low-speed authority that below a certain airspeed, there isn't enough aileron/spoileron roll authority to overcome the adverse roll from a hardover rudder.  Worse yet, the 737 PCU problem not only sent the rudder hard over, it caused a reversal to occur -- the pilots stepped on the pedal they thought would make things better, and the rudder problem got worse, confusing the absolute snot out of them.  You might be able to get out of it with differential power, provided it was applied soon enough to counter the roll before you went over on a wingtip and down...

Why would a rudder hard over cause the aircraft to roll?

@Au101 -- because rudder input causes a plane to roll as well as yaw (this coupling is related to the infamously annoying Dutch roll).

@Au101: Because rudder input causes the plane to yaw to one side, which causes the wing on the outside of the yaw to move faster than thee inside wing, which causes the outside wing to produce more lift than the inside wing, which lifts the outside wing relative to the inside wing, which causes the plane to roll in the direction of the rudder hardover.

Yes, diferential thrust saved this 747.  http://www.fss.aero/accident-reports/look.php?report_key=1012

@CamilleGoudeseune: Differential thrust plus having two rudders instead of just one.


text:
What is static discharging noise and what is its effect on radio communications?

What is static electricity discharging noise ?
How does this phenomena affect radio communications ?
Does the discharging happen through lightning and are static discharger  fixed or are they removable - such as telescopic i mean ?

The noise is created by sparks caused by static electricity. As the aircraft flies through the air, it picks up electrons from the air molecules. This extra charge will want to flow back into the air to equalize the charge. Small gaps in the conductive path on the aircraft will cause sparks to form. These sparks cause noise on radio frequencies, which can be picked up by the aircraft radios.
The static sparks will cause noise on the aircraft radios, sometimes a crackling sound.
Aircraft use static wicks on trailing edges to allow the charge to equalize. If components are electrically bonded well, static electricity should not be noticeable, other than under certain circumstances. The wicks are fixed and not telescopic but of course can be removed if replacement is needed.

@foot I asked if static wicks were removable just because I've never seen them, maybe it's too long time since I don't take a plane ;) and what about lightning in discharging ? Are there any incident caused by components improperly bounded ?

I remember seeing static wicks from my time in the Air Force, but since they weren't part of my job, I can't remember just when or where I saw them. If they were improperly bonded they'd probably fall off and the pilot would notice on his next walk around, if nobody else noticed first.

Static electricity is accumulation of charges over surface. During flight air frame of aircraft is exposed to the air and collision takes place between air molecules and air-frame which lead to development of electric charges over the air-frame. In my opinion water molecules on air-frame are also cause of static charge as water is slightly polar molecule it has slight negative charge on oxygen and slight positive charge on hydrogen.
Signals used in communication are electromagnetic in nature, i.e. they have electric as well as magnetic component. During interception of these communication signals with aircraft, static charges on air-frame interferes with the electric component of communication signal, hence cracking sound is there.
During lightening and thunderstorm lots of frequencies are produced sometimes these frequencies also interferes with the radio communication.


text:
What is the resolution of airport ground radars?
In largest airport a ground radar is used to handle safely aircrafts on taxiways and on runways, which should be the characteristics of this kind of radar ? What about its range and bearing resolution ?

It's been a while since my last lessons on radars, but..
The pulse length and the pulse repetition frequency play key roles in radar min and max ranges. The pulse the radar sends must be short enough to end (and the radar switch to detection mode) before the reflection returns.
To give a rough example, if the pulse length is 2 ms, the theoretical minimum range would be equal to pulse distance in 1 ms (1 there and 1 back).  The pulse travels at light speed, so you can calculate the distance there.
Note that the pulse lengths differ a lot depending on radars, and presumably none of the are 1 ms long.
Free quote from the book Understanding Radar Systems:

Pulse length of 3 µs, which is a fairly typical pulse length for a
  long-range airborne surveillance radar, would give a theoretical range
  resolution of 450 m. However, other factors such as signal processing
  degrade the resoution, and in the given case the resolution would be around 750 m.

Note again, that radars are designed to serve a specific purpose, for example a long-range surveillance radar emphasizes range over resolution.
What it comes to fighters, most modern fighters have AESA radars instead of pulse radars, which work continuously and handle the signal differently.

Some things I just can't Google while driving. Expect an edit later..

1 ms would be absurdly long. Such pulse would be 300 km long

I know. That's why i said "none of the are 1ms". That was given as an example, could have just as easily chosen "x".

For surface movement radars (SMR) operating in the 9 - 9,5 GHz band, the pulse length is typically in the order of 40ns. This is equivalent to 12 meters. That would bring the theoretical minimum range to 6 if we ignore transmitter / receiver switching overhead time.
In practice there is no need for such a short range. Instead a pulse train is transmitted before the antenna is switched from the transmitter to the receiver. This allows the radar to increase the accuracy by averaging multiple reflections, at the price of an increase in minimum range. Typical minimal range will be in the order of 100 meters.
Other SMR systems use continuous wave radar and have a transmitter and receiver antenna in the radar head. These systems don't require to switch between transmitting and receiving and don't have a minimum range.
The range resolution is in the order of 3 meters, equivalent to the pulse rise time of about 10ns.
The azimuth resolution of a typical SMR is in the order of 0.25 degrees, which is equivalent to 10 meters at 2 km distance.

Apart from primary radar there are other classes of surveillance systems in use for airport surface detection.

Secondary surveillance radar (SSR) lacks the accuracy of the SMR and
can only see transponders. But combined with the primary radar allows
for identification of aircraft on the airport surface (i.e. putting
identification labels on the aircraft's primary radar reflection).
Multilateration (MLAT) systems also rely on transponders but are more
accurate than SSR for surface monitoring. These systems require a large number of receivers and are susceptible for radio reflections from buildings.
ADS-B relies on the aircraft (or airport vehicle) transmitting its
position, speed and heading.

Don't ground radar systems use secondary radar instead  of primary radar for their use?  I believe that this is why the AIM now says to have the transponder on while taxiing and many large airports require it.

@Lnafziger SMR is primary radar, but often there is a SSR at/near the airport which is used for flight id / label correlation. The SSR alone is typically not able to provide the accuracy required for low visibility procedures on the surface.  Multilateration systems (like ASDE-X in the USA) depend on the transponder as well but can be much more accurate than a SSR.

@DeltaLima Would you expand on the meaning of the term "multilateration" in the context of your comment?  Is that the term for the multiple pulses you mentioned in the answer, or something else? Thanks

@RalphJ Multilateration is a surveillance technology that relies on the time difference of arrival of a radio signal at different positions. It consists of a number of radio receivers positioned carefully over the area under surveillance. The receivers listen to transmissions from the aircraft's transponder.  At a minimum there are 4 receivers, multilateration systems for airport surface surveillance typically have 20 or more.

@DeltaLima Interesting -- had no idea ASDE X did that.  So with all those receivers, it has a really precise location of the transponder antenna; is there also the radar as well to see other, non-transponder, vehicles on the airport surface?

@RalphJ ASDE-X combines several sensor systems, including SMR (which is primary radar) that can see non-transponder objects.


text:
What regulations govern the quality management system for producers of aircraft components?
I am looking for FAA documents and regulations regarding the quality management system for producers of aircraft components.
Specifically I would like to know what is a production certificate .
Furthermore may I download those regulation somewhere ?

From FAA, Production Certificate:

A production certificate is an approval (document) to manufacture
  duplicate products under an FAA-approved type design (e.g., type
  certificate or supplemental type certificate). The holder of a
  production certificate may obtain an airworthiness certificate for
  aircraft produced under the production certificate without further
  showing. To obtain one, the production application and approval
  process must be followed.

See also:
Production Approval - Regulations & Policies (link).

Are there any link to PDF download of these regulation ?
And please explain me that thing: I am company A (e.g. Boeing) who project and built the airplane, but I need a lot of components, so I have company B producing those components. Company B needs to have a production certificate right ? This certificate refers to one specific product or covers every product of certain categories for example ?
Hope being clear...!

Detail in subpart G. Also look at documents that are mentionned, e.g. Docket No. FAA-2006-25877. I guess the certificate may encompass multiple products, but this is likely explained in the process you need to follow for the request, as company B. I usually get pdf for 14 CFR at cornell.edu

It's all covered under 14 CFR Part 21.
Production Certificates provide authorization for production to holders of a Type Certificate - typically the aircraft or engine manufacturer. Suppliers to the Production Certificate holder can produce components under one of two types of authorization.
If the part being manufactured is defined by the the OEM's Type Certificate (an OEM part numbered drawing), they can produce it under a PMA - Parts Manufacturing Approval. (14 CFR Part 21, Subpart K).
If the part is covered by a TSO - Technical Standard Order, the supplier can produce the part under a TSOA - TSO Authorization.  (14 CFR Part 21, Subpart O).


text:
What techniques professional pilots use to keep their piloting skills sharp when not flying for an extended period?
I don't get to fly as often as I'd like. I know that simulators are good for brushing up on technique, learning new systems, etc.
Are there techniques that pros use (commercial or military) to stay sharp about things when they aren't able to actually fly? Specifically things that I would do not in a simulator, but say I have a couple months where I can't fly, but have an hour commute, or 30 minutes per day that I could do something flying-related to stay sharp.
I realize there is no substitute for actual flying, but I always feel a little rusty (mostly I just feel a little behind the airplane) when I haven't flown for a month or two. Are there things that other pilots do that I could learn to do in my down time that would mitigate this?

Reading a lot is key; staying on top of your checklists too. I noticed I'm behind the airplane more depending on the complexity of the aircraft in question. I keep photocopies of my checklists at home to refresh my mind prior to a flight.
It should also be mentioned the value of sitting in your aircraft on the ground; it costs nothing, and you can take your time working through checklists and scenarios in your mind.

I am not a professional but I'll offer up a few things.
I like to read here. Its actually quite a good place to learn things and see questions you might not otherwise ask yourself.
Read the FAR/AIM (or at least the most recent updates). This may seem boring but its a good way to stay current on new rules and regulations.
Fly the simulator. I like to fly the sim mainly for fun, but some times it's good just to remember what the instruments are shown when a variety of things happen. X-Plane now offers the ability to capture and use live weather. Plan a flight, then get in the sim with current conditions and see how good your wind calculations are.
Read your POH (if you own a plane or fly the same one frequently). Do you remember all your emergency checklists? All your operating procedures? Everything to check for preflight?
You would always buy a road atlas, get in your car, get yourself lost and try and find your way home using only paper maps and a compass. Might be a good navigation exercise.